1. Field of Invention
The present invention relates to a label printer that detects labels on label paper by an optical sensor, and to a method of detecting labels using an optical sensor.
2. Description of Related Art
Label printers that print on label paper having labels affixed at a fixed interval on a continuous label liner, and detect the labels and index the label paper (convey the label paper to the printing start position) using an optical sensor disposed to the transportation path through which the label paper is conveyed, are known from the literature.
Japanese Unexamined Patent Appl. Pub. JP-A-2001-180631 teaches a label printer that uses label paper having detection marks preprinted at equal intervals on the back side of the liner, and labels affixed at this equal interval to the liner so that the position of the edge of each label is aligned with the edge of a detection mark. This label printer emits light from the light-emitting device of an optical sensor to the back side of the label paper, and determines the position of a label by detecting the position of a detection mark based on the change in light reflected or transmitted from the label paper when the detection mark passes the detection position of the optical sensor. As a result, an evaluation value (threshold value) corresponding to the amount of reflected or transmitted light detected by the optical sensor when the detection mark passes is preset in memory, and the detection marks are detected by determining if the signal output from the photodetector of the optical sensor is less than or equal to this threshold value.
Labels can also be detected without using such detection marks by using the difference in reflectance or transmittance between parts where labels are affixed to the liner (that is, the label part where both label and liner are present) and parts where the labels are not affixed (that is, the liner part where only the liner is present), setting the amount of light reflected or transmitted from the label part or the liner part as a threshold value, and comparing this threshold value with the output signal from the photodetector of the optical sensor.
However, with a method that sets the absolute value of the light reflected or transmitted from the detection mark or label part as a threshold value, the output values will differ when label paper with reflectance or transmittance that is different from the anticipated label paper is used, and accurate detection may not be possible. As a result, only the label paper that can be accurately detected is identified as recommended paper, and the choice of label paper that can be used is limited. This is, however, not user-friendly for users that want to use various other types of label paper.
Setting threshold values for detecting plural types of label paper is also conceivable, but in this situation the threshold value ranges are extremely narrow and the possibility of detection errors caused by noise increases.
Resetting the threshold values so that using label paper with different reflectance or transmittance characteristics can be used is also conceivable, but in this situation the appropriate threshold value must be reset whenever the type of label paper changes, and reconfiguring the detection device is time consuming. Configuration errors are also possible.
Furthermore, with a method that sets the absolute value of the reflectance or transmittance as the threshold value, the effect of variation in the sensitivity of the photodetector is great and accurate detection can be difficult when the difference in the reflectance or the difference in the transmittance of the label parts and the liner parts is small, or when the amount of reflected or transmitted light that is detected is small.